Recently, semiconductor devices which use a metal oxide semiconductor thin film have been drawing attention. This thin film can be deposited at low temperatures and has characteristics such as a large optical band gap and optical transparency to visible light. Such a thin film can also form a flexible, transparent, thin film transistor (TFT) on a plastic substrate or a film or similar substrate.
For example, technology relating to a TFT in which an amorphous oxide film containing indium, zinc and gallium is employed for the channel layer (active layer) is disclosed in Nature, Vol. 432, 25 Nov. 2004 (488-492).
Further, the use of an oxide thin film which uses indium oxide as a main component for a TFT channel layer is disclosed in the Journal of Non-Crystalline Solids, 352, (2006), 2311. However, the atomic composition ratio (O/In) of the indium and oxygen in the oxide thin film is about 2.7, which is far deviated from the chemical stoichiometric ratio of 1.5.
The use of an oxide thin film which uses indium oxide as a main component for a TFT channel layer is also disclosed in Nature materials, VOL. 5, November 2006, (893-900). The indium oxide film is formed by ion-assisted deposition, and a thermal silicon oxide film and an organic thin film are used for the gate insulating film.
Although the TFT disclosed in Nature, VOL. 432, 25 Nov. 2004 (488-492) has a low current on/off ratio of about 103 it has a comparatively high field-effect mobility of 6 to 9 cm2/Vs, and is thus expected to be applied for a desirable active matrix in flat panel displays using liquid crystals or electroluminescence. However, with this TFT, various elements including indium, zinc, gallium and oxygen are used as the main constituent elements of the amorphous oxide film used for the channel layer. Thus, the TFT characteristics vary considerably depending on the composition.
From the standpoint of composition ratio controllability, the types of oxide composition elements are preferably as few as possible.
On the other hand, the TFT using indium oxide thin film for the channel layer disclosed in the Journal of Non-Crystalline Solids, 352, (2006), 2311, has a low current on/off ratio of about 104 and a field-effect mobility of about 0.02 cm2/Vs. As a result, this TFT is not suitable for high-speed operations and hence can be used in limited applications.
In the TFT using an indium oxide thin film for an channel layer disclosed in Nature materials, VOL. 5, November 2006, (893-900), an organic thin film having a high dielectric constant is used for the gate insulating film. The obtained TFT exhibits excellent characteristics, with an S-value (“S-value” being the gate voltage in the subthreshold region wherein the drain current is varied by one order of magnitude with the drain voltage constant) of 0.09 to 0.15 V/decade and a field-effect mobility of 120 to 140 cm2/Vs. However, as discussed above, because the gate insulating film is formed from an organic material, there is the problem that its environmental stability are lower than that of a TFT formed from an inorganic material. Further, while Nature materials, VOL. 5, November 2006 describes a TFT which uses thermal silicon oxide for the gate insulating film, and although the field-effect mobility in this case is about 10 cm2/Vs, which is comparatively high, regarding the subthreshold characteristics it has a large S-value of 5.6 V/decade. Therefore, its applications as a switching TFT are limited.